1. Field of the Invention
This invention relates chiefly to a beam splitter for use in an optical information recording-reproducing apparatus for optically or magneto-optically recording and/or reproducing information.
2. Related Background Art
In information recording-reproducing apparatuses for effecting the recording and/or reproduction of information by the use of light or light and magnetism, recording mediums in the form of a disk, a card or a tape have heretofore been used as recording mediums therefor. Among these optical or magneto-optical information recording mediums, there are widely known ones capable of recording and reproduction and ones capable of reproduction only. To record information on a medium capable of recording, a light beam modulated in accordance with recording information is condensed into the shape of a minute spot on said medium, and this spot is scanned along information tracks on said recording medium. Thereby, optically detectable information pit trains are formed on said recording medium.
Also, to reproduce information from said recording medium, a light beam of such a degree of low constant power not enough to effect recording on said-recording medium is condensed on said recording medium to thereby form a minute spot thereon, and the information pit trains on the information tracks are scanned by this minute spot. As a result, the reflected light or transmitted light from said recording medium is separated through an optical system and is read out.
The optical system used for the recording and/or reproduction of information on the above-described recording medium is equipped with an optical head which can be moved in the direction of the information tracks on the recording medium and a direction across that direction, and by the use of an objective lens provided on said optical head, a minute spot is formed on the recording medium from said light beam, and the information tracks on said recording medium can be scanned with the minute spot. Said objective lens is held on said optical head so as to be independently movable in the direction of the optical axis thereof (the focusing direction) and a direction orthogonal to both the direction of the optical axis and the direction of the information tracks on the recording medium (the tracking direction). Usually, a resilient member is used for such holding of the objective lens, and the movement of said objective lens in the two directions is effected by the driving of an actuator utilizing a magnetic interaction.
Among the above-mentioned optical information recording mediums, particularly the card-like optical information recording medium (hereinafter referred to as an optical card) is expected to have a big demand in the future as an information recording medium which is light in weight and convenient for carrying and has a relatively great capacity. The construction of such an optical card is generally as shown in FIGS. 1 and 2 of the accompanying drawings. That is, in FIG. 1, the optical card 1 has a number of information tracks 2 parallel-arranged in directions shown by arrows L-F on the information recording surface thereof, and a home position 3, which is the reference position of the access to the information tracks 2, is provided on said information recording surface. The information tracks 2 are arranged in the order of 2-1, 2-2, 2-3, . . . in succession from the home position 3, and as shown in FIG. 2, adjacent to these information tracks 2, tracks for tracking are provided in the order of 4-1, 4-2, 4-3, . . . These tracks 4 for tracking are used as a guide for auto-tracking (hereinafter referred to as AT) which controls the light beam spot so as not to deviate from a predetermined information track during the recording and/or reproduction of information.
In the optical system, a servo system for this AT functions to detect the deviation of said light beam spot from the information track (AT error), to negatively feed back the detection signal to the actuator, to move said objective lens in the tracking direction (D direction in FIG. 1) and to cause the light beam spot to follow a desired information track.
On the other hand, auto focusing (hereinafter referred to as AF) is effected to make the light beam spot on the surface of the optical card into a suitable size (focus the light beam spot) as long as the information tracks are scanned by the light beam spot during the recording and/or reproduction of information. In the optical system, a servo system for this AF functions to detect the deviation of the light beam spot from the in-focus state (AF error), to negatively feed back the detection signal to the actuator for focusing, to move the objective lens in the focusing direction and to focus the light beam spot on the surface of the optical card.
Therefore, in the optical system, the light beam applied to the optical card is divided into three light beams, which are made into three light beam spots S1, S2 and S3, respectively, on the optical card, as shown in FIG. 2. Tracking control for the information tracks is effected by the use of the light beam spots S1 and S3, and focusing control, the production of information pits during recording and the reading-out of the information pits during reproduction are effected by the use of the light beam spot S2. In the respective information tracks shown in FIG. 2, reference characters 6-1, 6-2, and 7-1, 7-2 designate preformed left address portions and right address portions, and by these address portions being read out during the recording and/or reproduction of information, discrimination between the information tracks is effected. Also, in FIG. 2, reference characters 5-1 and 5-2 denote predetermined information recorded on a data portion.
As is well known, there are broadly two kinds of optical information recording systems using the above-described optical system. One of them is a single-light-source system which effects recording and reproduction by one and the same light source, and the other is a two-light-source system which effects recording and reproduction by two different light sources. The two-light-source system, as compared with the single-light-source system, is said to have the advantages that the reproducing light is not deteriorated and the system is suited for the realization of a higher speed.
The optical system of the conventional two-light-source system is of a construction as shown in FIG. 3 of the accompanying drawings in which recording light and reproducing light are supplied from discrete light sources, whereby deterioration of the reproducing light is prevented and high-speed recording is made possible. In FIG. 3, a semiconductor laser 21 as a light source emits light of a wavelength of 780 nm and another semiconductor laser 22 emits light of a wavelength of 830 nm. In FIG. 3, reference numerals 23 and 24 designate collimator lenses, reference numeral 25 denotes a diffraction grating for dividing a light beam, reference numeral 26 designates a dichroic prism designed to transmit therethrough light of 780 nm which is a P-polarized component and reflect light of 830 nm, and reference numeral 27 denotes a polarizing beam splitter (a beam shaping prism). Reference numeral 29 designates a quarter wavelength plate, reference numeral 30 denotes an objective lens, reference numeral 32 designates a light intercepting plate, reference numeral 33 denotes a toric lens, and reference numeral 34 designates a photodetector.
Light beams emitted from the semiconductor lasers 21 and 22 are divergent light beams and therefore, pass through the collimator lenses 23 and 24, whereby they are converted into parallel light beams. The light of 780 nm from the semiconductor laser 21 further enters the diffraction grating 25, and is divided into three effective light beams (0-order diffracted light and .+-.1st-order diffracted light) by the function of this diffraction grating. The light beam of 780 nm and the light beam of 830 nm from the semiconductor laser 22 enter the dichroic prism 26 having a necessary spectral characteristic, and the light of 780 nm which is P-polarized light is transmitted through the dichroic prism and the light of 830 nm is reflected by the dichroic prism, and the two light beams in their combined state emerge from the dichroic prism 26. The light beam passed through this dichroic prism 26 enters the polarizing beam splitter 27 having the light splitting function. The polarizing beam splitter 27 has such a spectral characteristic that transmits P-polarized light and reflects S-polarized light, but the light beams of two wavelengths are P-polarized light components and are therefore transmitted through this polarizing beam splitter. The polarizing beam splitter 27 is three-point-supported relative to an optical head housing (not shown) by abutting portions P.sub.1, P.sub.2 and P.sub.3. The polarizing beam splitter 27 is comprised of three prisms.
These light beams of two wavelengths are then converted into circularly polarized lights when they are transmitted through the quarter wavelength plate 29, and are converged on the optical card 1 which is the information recording medium by the objective lens 30. The light beam of 780 nm, in the form of three minute beam spots S1 (+1st-order diffracted light), S2 (0-order diffracted light) and S3 (-1st-order diffracted light), is applied onto the optical card 1, and spot S2 is used as reproducing light and a light signal for AF control, and spots S1 and S3 are used as light signals for AT tracking. Also, the light beam of 830 nm, in the form of a minute beam spot S2 (0-order diffracted light), is applied onto the optical card 1 and is used as recording light.
In this case, the positions of the light beam spots on the optical card 1 are similar to those in FIG. 2, that is, the light beam spots S1 and S3 are positioned on adjacent tracks 4 for tracking and the light beam spot S2 is positioned on the information track 2 between those tracks 4. Also, the positional relation between the light beam spot S2 of 780 nm and the light beam spot S2 of 830 nm is free in principle and here, these two light beam spots are positionally coincident with each other. Thus, the reflected light from the light beam spots formed on the optical card 1 passes through the objective lens 30 and becomes a substantially parallel light beam, and is again transmitted through the quarter wavelength plate 29 and becomes a light beam having its direction of polarization rotated by 90.degree. from that during the incidence. Therefore, the reflected light enters the polarizing beam splitter 27 as an S-polarized light beam. However, the polarizing beam splitter 27 has the characteristic of reflecting S-polarized light, as previously described. Here, the reflected light from the light spot of 830 nm is reflected and directed to the light intercepting plate 32. Also, the reflected light from the light spot of 780 nm is converged by the toric lens 33 and enters the photodetector 34. The photodetector 34 is comprised, for example, of two light receiving elements and a four-division light receiving element, and a signal for tracking control is provided by the former and a signal for focusing control and reproduction is provided by the latter.
What poses a problem here is that in the aforedescribed information recording-reproducing apparatus for optically or magneto-optically effecting the recording and/or reproduction of information, to accomplish stable recording and/or reproduction or AT/AF control, high manufacturing and assembling accuracy of optical parts in the optical system is required. Particularly, in the aforedescribed polarizing beam splitter 27 constructed by a combination of a plurality of optical parts (glass-molded articles), the cementing accuracy of the optical parts, in addition to the fixing accuracy, must be high.
Therefore, there are known the following two methods in the cementing work for the polarizing beam splitter. One method is a method as shown in FIG. 4 of the accompanying drawings wherein when three prism members A, B and C are to be superposed one upon another so as to have the light splitting function on the cemented surfaces thereof, the sides A.sub.1, B.sub.1 and C.sub.1 thereof opposite to a location in which the vertex angles of the respective prism members concentrate (point Y in FIG. 5) are abutted against a reference plane X and in that state, the prism members are cemented together. The other method is a method as shown in FIG. 5 of the accompanying drawings wherein the vertex angles of the respective prism members are concentrated at point Y and the prism members are cemented together.
However, when the prism members are cemented together by any of such methods, if there exist working errors of the respective prism members, dimensional error during the abutting of the prism members, irregularity of the thicknesses of an adhesive agent, etc., there will occur an angular error attributable to the shift of each prism member or the mutual pressing among the prism members.
That is, if as shown in FIG. 6 of the accompanying drawings, a working error exists in the prism member B and the prism member B is smaller than the design value, a gap E will be created between the prism members A and B.
In order to eliminate such a gap E, it would occur to mind to shift each prism member as shown in FIG. 7 of the accompanying drawings, concentrate the vertex angles of the respective prism members at a point Y and cement the prism members together. In such a case, however, the prism member A would be inclined. This would lead to the deviation of the emergence position and the deviation of the emergence angle of the beam from a constituent member as a polarizing beam splitter, and would finally bring about a fluctuation in the AT/AF control signal.
Description will now be made of a problem arising when the polarizing beam splitter 27, in which an angular error attributable to the shift of each prism member or the mutual pressing among the prism members has occurred as described above, is supported.
If as shown in FIGS. 8 and 9 of the accompanying drawings, abutting portions P.sub.1, P.sub.2 and P.sub.3 as shown are adopted in a three-point-support, the polarizing beam splitter 27 in which such angular error has occurred on an optical head housing, actual transmitted light (indicated by a solid line in FIG. 8) will be shifted relative to the ideal optical axis (indicated by a dotted line in FIG. 8) or will have an angular error.